The present invention relates to improvements in apparatus for applying looped webs or bands around various objects, especially around parcels, packages and similar block-shaped commodities. More particularly, the invention relates to improvements in apparatus of the type wherein a band or web of weldable synthetic plastic or other suitable material is applied (looped) around an object, wherein the resulting loop is separated from the remainder of the web in such a way that the two end portions of the loop overlie each other, and wherein the overlapping end portions of the loop are thereupon connected to each other to form a joint.
It is already known to form a closed loop around a block-shaped package or an analogous object in such a way that neither of the two overlapping end portions of the joint extends from the looped web, i.e., that the outer overlapping portion is fully bonded to the adjacent inner portion of the looped web. This is desirable because the projecting part of a web is likely to be caught during transport or during other manipulation of the object and to cause damage to or complete destruction of the joint.
In presently known apparatus for applying loops of web-like material around parcels or the like, the joint is formed by welding the normally thermoplastic synthetic plastic material of the web. Thus, the overlapping end portions of the loop are heated to a temperature at which the material of such portions melts, and the resulting welded joint is allowed or caused to set in order to prevent opening of the loop. The knife which severs the loop from the remainder of the web (e.g., from a roll of convoluted web) is mounted on a first welding electrode which is movable toward and away from a stationary counterelectrode. The overlapping portions of the loop are disposed in the space between the two electrodes. The severing takes place simultaneously with bonding or immediately prior to heating of overlapping portions of the loop. In many instances, the knife is closely adjacent to the mobile electrode so that it can sever the loop in immediate proximity to the welded joint.
The just described prior art apparatus are satisfactory to the extent that they insure the absence of projecting portions in the region where the end portions of the loop overlie and are bonded to each other. However, thermal welding takes up a considerable amount of time, not only because the overlapping portions of the web must be heated at least close to the melting point but also because the joint must be allowed to set in order to avoid its destruction immediately after the completion of the thermal welding operation. Thus, the object to be provided with a looped web must remain in the apparatus until the material of the joint sets.
It is also known to weld synthetic plastic material by resorting to an ultrasonic welding unit. Such mode of welding requires less time than thermal welding because the joint is sufficiently strong immediately upon completion of the welding operation and the making of the joint also requires less time than the making of joints by resorting to thermal welding. Ultrasonic welding apparatus employ so-called sonotrodes which must be operated in the resonance range in order to guarantee an economical ultrasonic welding operation. The resonance of the sonotrode can be adversely affected by the knife if the latter contacts the sonotrode during the making of an ultrasonically formed welded joint.